Hook up tho positive side of the battery. Be sure everything is turned off. Place a test light between the negative post and cable. If the light comes on, you have a drain or short.
If not proceed to check battery draw (amps) as you turn on various switches.
A dead short should at least blow a fuse if not let the smoke out of the wires.
A heavy starter draw can drain the battery fast, but the cables would get very hot.
A bad ground would show up as hot wires also.
You may just be getting junk batteries. (you do charge them before you hook them up don't you?)
